OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. OLED technologies are reviewed in Geffroy et al., “Organic light-emitting diode (OLED) technology: material devices and display technologies,” Polym., Int., 55:572-582 (2006). Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
In many cases, a large portion of light originating in an emissive layer within an OLED does not escape the device due to internal reflection at the air interface, edge emission, dissipation within the emissive or other layers, waveguide effects within the emissive layer or other layers of the device (i.e., transporting layers, injection layers, etc.), and other effects. In a typical OLED, up to 50-60% of light generated by the emissive layer can be trapped in a waveguide mode, and therefore fail to exit the device. Additionally, up to 20-30% of light emitted by the emissive material in a typical OLED can remain in a glass mode. The out-coupling efficiency of a typical OLED, thus, can be as low as about 20%. See, for example, U.S. Patent Application Publication No. US 2008/0238310 A1, which is incorporated herein by reference in its entirety.
Organic OLED layers, in a conventional process, are deposited by vacuum thermal evaporation (VTE). In such an OLED deposition process, organic layers are usually deposited at a slow deposition rate (from 1 Angstroms/second to 5 Angstroms/second) and the deposition time is undesirably long when a thick buffer layer is deposited. Moreover, to modify the layer thickness, a stencil mask is usually applied to either shield the vapor flux or allow it to pass through onto the substrate. Furthermore, the resulting organic layer usually has an index of refraction that is significantly higher than the index of refraction of the substrate glass. As such, some of the emitting light can be trapped in the organic layer and lost in a waveguide mode.
In vacuum thermal evaporation (VTE), a layer of hole transport material (HTM) is deposited in a vacuum at slow rate, for example, at about 0.2 nm/sec for 110 nm at a total time of about 550 seconds or about 9.0 minutes. No conventional technique, such as VTE, for forming organic thin films combines the large area patterning capabilities of inkjet printing with the high uniformity, purity, and thickness control achieved with vapor deposition. Given that inkjet-processed single layer OLED devices continue to have inadequate quality for widespread commercialization, and thermal evaporation remains impractical for scaling to large areas, it is desired to develop a technique that can offer both high film quality and cost-effective large area scalability.